DETROIT (Reuters) – The drinking water in Flint, Michigan, where high lead levels led to a health crisis that drew national attention, is improving, but remains unstable, a top environmental official said Friday.

“The drinking water system is recovering,” Robert Kaplan, the U.S. Environmental Protection Agency’s acting administrator for the region that includes Flint, told local and state officials meeting in the city to discuss the crisis.

“You’ve got a dramatic decrease in the soluble lead. What we’re seeing though is particulate lead, which indicates that the system is unstable,” he told the meeting by phone.

Under the direction of a state-appointed emergency manager, Flint switched water supplies to the Flint River from Detroit’s system in 2014 to save money. The state has been criticized for its initial poor handling of the issue.

The corrosive river water leached lead, a toxic substance that can damage the nervous system, from the city’s water pipes. Flint switched back to the Detroit system last October.

“Whenever we see a positive trend in Flint’s water quality, that’s good news, but we still have much work to do to get people the quality of water they need and deserve,” Michigan Governor Rick Snyder said in a statement on Friday.

Kaplan said while the addition of chemical phosphates to recoat the pipes to inhibit corrosion is working, the almost invisible lead particles remain a random and unpredictable problem.

Kaplan said water filters reliably deal with the lead, but the best approach would be for residents to vigorously flush their home water systems by turning on all faucets and spigots and running the water to clean the sediment out and rebuild the protective phosphate coating.

He said local and state officials need to have a simple message for residents in the city of 100,000 people to take that approach.

“If we don’t have an extremely simple message, as in free water, you will not be charged for the water that you use that is related to this flushing, I’m afraid we’re not able to get that lead washed out of the system,” he said.

The state previously approved $30 million to help Flint residents pay their water bills dating back to when the switch to the Flint River was made.

Kaplan said a full recovery of Flint’s water system will take time, adding experts would not provide a time table at this point.

(Reporting by Ben Klayman; Editing by Alistair Bell)